Fixed-Disk System
The fixed-disk system, described in the United States Pharmacopeia (USP), includes a steel punch, matrix, neoprene ring, surface plate, and support. The system requires a support to sustain the matrix and the punch on a vessel with a flat bottom, ensuring that the area of the drug is fixed and exposed upwards. The rotational movement is controlled by a paddle of Apparatus 2. This configuration ensures that the exposed surface area of the drug is consistent throughout the test.
Rotating-Disk System (Wood’s Apparatus)
The rotating-disk system, known as Wood’s apparatus, is described in the USP, as well as in the European and British Pharmacopeias. This system consists of a steel punch, matrix (die), spindle, neoprene ring, and surface plate. The surface plate includes holes for fastening the matrix screws, facilitating the fabrication of the compressed drug. The matrix contains a cavity where the drug and punch are inserted and compressed. The material is then exposed on the face of the matrix, which is coupled to a spindle that controls its movement.
Characteristics and Use
These apparatuses are used in conventional dissolution equipment and have a cavity for placing the drug. A press is required to form the compressed drug, with known geometry and surface area. Once placed in the dissolution equipment, these systems ensure that the compressed drug is exposed in an area with lower hydrodynamic variability. The fixed-disk system maintains the area of the drug fixed upwards, while the rotating-disk system involves rotational movement controlled by a spindle.
Mini-IDR and Other Systems
Other systems not detailed in pharmacopeias but used in various studies to determine the IDR include the flow-cell system and a miniature apparatus similar to the rotating disk, known as the Mini-IDR. These alternative systems offer flexibility in determining the IDR of low-solubility drugs and provide comparable results to the standard fixed and rotating-disk systems.
Calculating Intrinsic Dissolution Rate (IDR)
To calculate the IDR, an equation derived from the Noyes–Whitney equation is used. The formula for IDR calculation is:
j=V⋅(dc/dt)/A
where:
- j is the dissolution flow (mg cm-2 s-1),
- V is the volume of the dissolution medium (mL),
- dc/dt is the concentration change of the dissolved drug in the medium over time (mg/mL),
- A is the surface area of the sample (cm2).
The IDR is calculated by plotting the cumulative amount of dissolved drug against time and performing a linear regression on the data. The slope of the initial linear portion of this plot gives the dissolution rate in units of mass per time. The intrinsic dissolution rate is then obtained by dividing this value by the surface area of the compressed drug and is expressed in units of mass per area per time (mg cm-2 s-1).
Read also: USP Dissolution Apparatus | Types and Principles
Resource Person: Prakash Amate
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